Jenny Hoffman

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Relating atomic-scale electronic phenomena to wave-like quasiparticle states in superconducting Bi₂Sr₂CaCu₂O_8+x

K. McElroy,^1,2 R. W. Simmonds,¹ J. E. Hoffman,¹ D.-H. Lee,^1,2,3 J. Orenstein,^1,2 H. Eisaki,⁴ S. Uchida,⁵ J. C. Davis^1,2,6
1Department of Physics, University of California, Berkeley, California 94720-7300, USA
²Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
³Center for Advanced Study, Tsinghua University, Beijing 100084, China
⁴AIST, 1-1-1 Central 2, Umezono, Tsukuba, Ibaraki, 305-8568 Japan
⁵ Department of Superconductivity, University of Tokyo, Tokyo, 113-8656 Japan
⁶Department of Physics, LASSP, Cornell University, Ithaca, New York 14850, USA

Nature Volume 422, 592-596 (10 April 2003).

Abstract

The electronic structure of simple crystalline solids can be completely described in terms either of local quantum states in real space (r-space), or of wave-like states defined in momentum-space (k-space). However, in the copper oxide superconductors, neither of these descriptions alone may be sufficient. Indeed, comparisons between r-space and k-space studies of Bi₂Sr₂CaCu₂O_8+x Bi-2212) reveal numerous unexplained phenomena and apparent contradictions. Here, to explore these issues, we report Fourier transform studies of atomic-scale spatial modulations in the Bi-2212 density of states. When analysed as arising from quasiparticle interference, the modulations yield elements of the Fermi-surface and energy gap in agreement with photoemission experiments. The consistency of numerous sets of dispersing modulations with the quasiparticle interference model shows that no additional order parameter is required. We also explore the momentum-space structure of the unoccupied states that are inaccessible to photoemission, and find strong similarities to the structure of the occupied states. The copper oxide quasiparticles therefore apparently exhibit particle–hole mixing similar to that of conventional superconductors. Near the energy gap maximum, the modulations become intense, commensurate with the crystal, and bounded by nanometre-scale domains4. Scattering of the antinodal quasiparticles is therefore strongly influenced by nanometre-scale disorder.

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For some perspective on this issue you can read the News and Views article about this paper by Jan Zaanen:
"Superconductivity: Pebbles in the nodal pond"
Nature 422, 569-570 (10 April 2003).